Interpretive Summary: ARS scientists have developed and released corn germplasm that resists attacks by two major lepidoptera pests, the southwestern corn borer and the fall armyworm. The objectives of the present research are to determine if this germplasm that is resistant to newly hatched larvae is also resistant to older larvae of both pest species; and if larvae initially fed on resistant tissue can recover from its effects when provided susceptible plant tissue. The experimental results showed that the resistant corn hybrid is resistant to not only newly hatched larvae, but also older southwestern corn borer and fall armyworm larvae based on growth and development of different age larvae fed selected regimes of excised whorl leaf tissue from a resistant and a susceptible hybrid under laboratory conditions. Additionally, larvae of both species recovered from feeding on resistant tissue when switched to susceptible tissue. This information provides a better understanding of the nature of the resistance and evidence that resistance to older larvae (3 to 7 days old) does occur within corn.

Technical Abstract:
Maize, Zea mays L., germplasm with resistance to leaf feeding by southwestern corn borer, Diatraea grandiosella Dyar, and fall armyworm, Spodoptera frugiperda (J. E. Smith) neonates has been developed and released. Laboratory studies were conducted during two years to determine if this resistance extends to older larvae and if larvae initially fed resistant tissue recover when switched to susceptible tissue. Developing larvae of both species were fed selected regimes of resistant and susceptible tissue from field grown whorl stage plants to evaluate for a resistance or a recovery response by the insects. Biological responses to the feeding regimes were obtained by weighing larvae at specified time intervals. Additional data obtained for the fall armyworm were pupal weights, developmental times to pupation and adult eclosion, and adult morphological normality. Results indicated a resistance and a recovery response to larvae of both species depending upon tissue feeding regime. The most interesting response occurred with fall armyworm fed resistant tissue after being initially fed susceptible tissue for 7 d. Although the larvae fed the susceptible and then the resistant tissue gained less weight after the tissue switch than those reared only on susceptible tissue, the number of days to pupation did not differ. However, the pupae of those fed susceptible tissue for 7 d and then resistant tissue weighed only 138.5 mg compared to 182.1 mg for those reared on only susceptible tissue.